How many liters of red water should be added to 15 liters of a 35 percent solution to get a 25% solution.

1) Determine the mass of the substance in the initial solution.

To do this, multiply 15 liters of the solution by the percentage of the substance in it.

We get:

15 * 35% / 100% = 15 * 0.35 = 5.25 liters.

2) We make up the proportion in which we write the new volume of the solution as x liters.

In this case, x liters would refer to 100% in the same way as 5.25 liters of a substance would refer to 25%.

We get:

x / 100% = 5.25 / 25%.

25 * x = 525.

x = 525/25 = 21 liters.

3) Find how much to add.

21 – 15 = 6 liters.

Answer: It is required to add 6 liters.